1. Field of the Invention
The invention relates to a method of dry etching, and more particularly to a method of dry etching a nickel film for making an electrode. The invention also relates to an apparatus of making dry etching exhaust gas non-toxic.
2. Description of the Related Art
There has been used various methods and apparatuses for dry etching of nickel in order to etch a nickel film with high accuracy to thereby form an electrode in a semiconductor device.
For instance, Japanese Unexamined Patent Publication No. 53-20769 has suggested a method of dry etching a nickel film by means of plasma generated from CO or CO.sub.2 gas. The suggested method utilizes a method of refining metal, known as carbonyl process. Iwanami, Scientific and Chemical Encyclopedia, the 3rd edition, pp. 270 defines the word "carbonyl process" as follows: "This is a method of refining metal, utilizing thermal dissociation of metal carbonyl. Mond process for refining nickel is an example of the carbonyl process, and is based on the following equilibrium equation. EQU Ni(CO).sub.4 Ni+4CO
If nickel dioxide (II) were reduced to metal nickel with hydrogen gas, the thus reduced metal nickel were caused to react with CO at 60.degree. C. to thereby produce gaseous nickel carbonyl, Ni(CO).sub.4 having a boiling point of 42.5.degree. C., and then the thus produced gaseous nickel carbonyl were heated up to 180.degree. C. in a decomposition tower, the above-mentioned equilibrium equation proceeds towards the right. As a result, there is obtained highly purified nickel in the form of powder, including CO in a small content."
As mentioned above, Ni can be etched merely by causing Ni to directly react with CO gas. The above-mentioned Japanese Unexamined Patent Publication No. 53-20769 further suggests a method of etching Ni by producing plasma of CO gas, but does not mention an advantage of producing plasma of CO gas.
When CO.sub.2 gas is employed, CO.sub.2 gas has to be turned into plasma in order to etch Ni. This is because CO.sub.2 gas is decomposed by being turned into plasma to thereby produce CO gas necessary for etching Ni.
By etching Ni with either CO or CO.sub.2 gas, there can be obtained an advantage that it is no longer necessary to use chemicals which has a problem that waste solution thereof is quite difficult to properly dispose of.
Japanese Unexamined Patent Publication No. 60-228687 has suggested another method of dry etching nickel by using CO.sub.2 plasma. The Publication mentions that a method of etching nickel at a temperature in the range of 40.degree. C. to 100.degree. C. by using CO gas has a shortcoming of poor etching accuracy relative to a mask pattern due to that etching is isotropic. In addition, the Publication further mentions that if plasma were generated based on CO gas, there would be generated O.sub.2 gas which would oxidize nickel, and hence make it difficult to carry out nickel etching. As a solution to such a problem, the Publication has suggested the use of CO.sub.2 plasma for etching nickel, and addition of H.sub.2 to CO.sub.2 gas for preventing oxidation of nickel.
Japanese Unexamined Patent Publication No. 60-228687 presents a graph showing a relation between a nickel etching rate and input power provided to a discharge electrode, which relation is found when nickel is etched by means of plasma of CO.sub.2 gas. The graph is illustrated in FIG. 1. According to FIG. 1, an input power of about 150 W is required for obtaining an etching rate of 1000 angstroms/min., for instance.
For the purpose of dry etching nickel, there may be used a variety of etching apparatuses. For instance, there may be used a plane parallel plate type reactive ion etching (RIE) apparatus, an electron cyclotron resonant (ECR) etching apparatus, or an inducive coupling type plasma (ICP) etching apparatus.
FIG. 2 illustrates an exhaust system of those etching apparatuses. The exhaust system comprises an exhaust pump system 80 disposed downstream of an etching apparatus 79, and an apparatus 81 for making an exhaust gas non-toxic, disposed downstream of the exhaust pump system 80. The exhaust gas 98 made non-toxic by the apparatus 81 is released to atmosphere, or introduced to a gas scrubber (not illustrated) through an exhaust conduit 97. As illustrated in FIG. 2, the exhaust pump system 80 generally includes a booster pump or turbo-molecular pump 95, and a rotary pump employing oil for rotation or a dry pump 96 employing no oil for rotation, disposed downstream of the pump 95.
The etching apparatus 79 includes an etching chamber 88 having a gas inlet 85 through which etching gas 84 is introduced into the etching chamber 88 The etching chamber 88 is provided with a gate 83 which is designed to be open or closed by means of a gate valve 82. Inside the etching chamber 88 are disposed an anode 89 and a cathode 90. The anode 89 is supported in the etching chamber 88 with an insulator 86, and is grounded at 87. The cathode 90 is supported in the etching chamber 88 with an insulator 99 in the same fashion as the anode 89, and is electrically connected to RF power source 93 via a matching box 92. The RF power source 93 is grounded at 94. A substrate 91 on which a nickel film to be etched is formed is placed on the cathode 90.
However, the above-mentioned conventional methods of dry etching have problems as follows.
The first problem is that nickel is isotropically etched in accordance with any one of the conventional methods, causing side etching with the result of poor accuracy in nickel etching relative to a mask pattern. This is because that CO gas directly reacts with Ni, which causes isotropic etching, in a conventional method where Ni is etched with CO gas at 40.degree. C. to 100.degree. C.
In a method of dry etching Ni with plasma of CO gas without keeping a substrate at 40.degree. C. or lower, a temperature of a substrate would be over 40.degree. C. by ion radiation to the substrate while nickel is being etched. Thus, as a result, etching is carried out isotropically due to direct reaction of CO gas with Ni.
Similarly, in a method of dry etching Ni with plasma of CO.sub.2 gas without keeping a substrate at 40.degree. C. or lower, a temperature of a substrate would be over 40.degree. C. by ion radiation to the substrate while nickel is being etched. As a result, etching is carried out isotropically due to direct reaction between CO generated by decomposition of CO.sub.2 gas in plasma and Ni.
The second problem is that when nickel is etched with plasma formed singly of CO gas, excessive deposition is accumulated on a surface of a substrate, which causes reduction in a nickel etching rate, or in some cases, inability of nickel etching. The reason is as follows. In decomposition of CO.sub.2 gas, there are found two stages. Namely, CO.sub.2 gas is first decomposed into CO and O in the first stage, and CO is further decomposed into C and O in the second stage. To the contrary, in decomposition of CO gas, CO gas is decomposed directly into C and O. Hence, non-volatile carbon is generated in a greater amount than CO.sub.2 gas, resulting in that deposition onto a substrate is generated in a greater amount.
The third problem is that plasma formed singly of CO gas and plasma formed singly of CO.sub.2 gas could not etch nickel without causing damage to an underlying substrate. The reason is as follows. If nickel is etched with plasma formed singly of CO gas or plasma formed singly of CO.sub.2 gas, nickel is oxidized at a surface thereof by O or O.sub.2 generated by decomposition of CO and CO.sub.2. Hence, if input power were decreased in order to prevent an underlying substrate from being damaged due to ion radiation, it would be impossible to remove a surface oxidation film of nickel, and as a result nickel could not be etched.
If nickel is etched with plasma of CO.sub.2 and H.sub.2 gases, nickel tends not to be oxidized at a surface thereof, which avoids inability of nickel etching caused by oxides formed on a surface of nickel. However, it would be necessary to increase input power to be provided to a discharge electrode in order to decompose CO.sub.2 gas by plasma to thereby sufficiently produce CO acting as an etchant for nickel. Hence, the plasma of CO.sub.2 and H.sub.2 gases is not allowed to be applied to etching where it is expected to avoid causing damage to an underlying substrate. For instance, an input power of about 150 W is necessary to be provided to a discharge electrode in order to obtain an etching rate of 1000 angstroms/minute. However, an input power of about 150 W would cause too much ion bombardment to an underlying substrate, resulting in that crystallinity in the underlying substrate would be destroyed. If an input power were reduced down to about 30 W in order to reduce damage to the underlying substrate caused by ion bombardment, an nickel etching rate would be reduced to smaller than 500 angstroms/minute, which is not a practical etching rate.
The fourth problem is that exhaust gas resulted from nickel etching was not always disposed properly for making harmless. The reason is as follows. As described in Iwanami, Scientific and Chemical Encyclopedia, the third edition, pp. 981, Ni(CO.sub.4) gas resulted from nickel etching is virulently toxic. However, Ni(CO.sub.4) gas is removed after it has passed through an exhaust pump, Ni(CO.sub.4) gas discharged from an etching chamber pollutes the exhaust pump. Thus, operators doing maintenance of the pump are exposed to serious danger. Such a danger is significantly serious in particular in an oil rotation type rotary pump which requires oil exchanges.